Isotropic Seebeck coefficient of aligned single-wall carbon nanotube films

Abstract

How the morphology of a macroscopic assembly of nanoobjects affects its properties is a longstanding question in nanomaterials science and engineering. Here, we examine how the thermoelectric properties of a flexible thin film of carbon nanotubes depend on macroscopic nanotube alignment. Specifically, we have investigated the anisotropy of the Seebeck coefficient of aligned and gated single-wall carbon nanotube thin films. We varied the Fermi level in a wide range, covering both the p-type and n-type regimes, using electrolyte gating. While we found the electrical conductivity along the nanotube alignment direction to be several times larger than that in the perpendicular direction, the Seebeck coefficient was found to be fully isotropic, irrespective of the Fermi level position. We provide an explanation for this striking difference in anisotropy between the conductivity and the Seebeck coefficient using Mott's theory of hopping conduction. Our experimental evidence for an isotropic Seebeck coefficient in an anisotropic nanotube assembly suggests a route toward controlling the thermoelectric performance of carbon nanotube thin films through morphology control.

Carbon and carbon composites for thermoelectric applications

“…Besides, Ichinose et al 32 suggested that the alignment of SWCNTs led to an enhanced σ but unchanged S for the aligned metallic film (sample #5), and a PF value of around 300 μW m −1 K −2 was achieved. Similar conclusion was also drawn in the study by Fukuhara et al 33 on SWCNT film (a mixture of metallic and semiconducting SWCNTs). The testing direction was illustrated in Figure 2F and SWCNT alignment degree was evaluated by polarization‐dependent Raman spectra (Figure 2G).…”

“…Reproduced with permission: Copyright 2018, American Institute of Physics. 33 K, S of the aligned SWNT sample before and after annealing. Reproduced with permission: Copyright 2000, American Institute of Physics 34 isotropic S according to Mott's theory.…”

“…H‐J, σ , S , and PF as a function of gate voltage in the parallel and perpendicular directions in an aligned SWCNT thin film. Reproduced with permission: Copyright 2018, American Institute of Physics 33 . K, S of the aligned SWNT sample before and after annealing.…”

Modulation of intertube band dispersion relation of carbon nanotube bundles by symmetry and intertube wave function coupling

“…Owing to these unique properties, carbon nanotubes are regarded as emerging materials in many modern technological areas such as flexible electrodes [12][13][14] , field effect transistors [15][16][17] , thermoelectric devices [18][19][20] , and reinforced materials [8][9][10][11] .…”

Structural effects on carrier doping in carbon nanotube thin-film transistors

“…At the intersectional and aligned regions, the intertube interaction plays decisive roles to determine the device performance, because the intertube interaction basically causes modulation of the electronic structure of the CNTs. Although the CNT mat films and aligned CNTs can work as a conducting channel in real devices, such as multi-stage ring oscillators 24,25 and thermoelectric devices [31][32][33] , little is known about the microscopic mechanism of carrier accumulation in CNT thin films by a gate electrode. In our previous work, we predicted that field inversion occurs at the intersectional region of two CNTs under a weak electric field 34,35 .…”